Heap leaching is an industrial mineral process to extract precious metals, such as copper, from ore via a series of chemical reactions that absorb specific target copper-bearing minerals with subsequent separation of these precious metals from other earth materials. The experience at heap leaching plants for copper oxides having nitrate (NO3−) salts found in copper ore deposits has shown that sulfuric acid (H2SO4) consumption is generally much lower than budgeted by design (acid consumption <50%), when nitrate levels have remained high in the copper ore deposits.
The main concern of copper producers was the damage caused by the presence of nitrate salts in the solvent extraction and electrowinning (i.e., electrodeposition), also known as electro extraction, stages, rather than the benefits from sulfuric acid savings effectuated when sulfuric acid consumption was less than design levels due the presence of nitrate salts. Different mixtures of reagents were used in the solvent extraction and re-extraction stages to ensure appropriate extraction and re-extraction efficiency. Likewise, lower doses of a polysaccharide “guar” additive (such as Galactasol, OPT-45, Guartec, and Guarfloc) in the electrowinning process allowed control of premature degradation of this polysaccharide due to nitrate ions (NO3−), resulting in massive formation of stripes over the cathodes in the electrowinning cell. This issue appeared frequently in plants where the control of impurity carryover from solvent extraction was not completely efficient and nitrate levels in the electrolyte solution exceeded 50 parts per million (ppm).
The analysis of the problem associated with nitrates in leaching of copper oxide plants, coupled with the inventors' industrial experience, indicate that the presence of nitrate ions in the leaching and electrowinning processes can be beneficial so long as there is an appropriate control of nitrate ion levels in the different aqueous solutions generated or used in the ore curing, leaching, or electrowinning processes.
Therefore, it is interesting to point out the chemical properties of nitrates as compared to sulfates (SO42−). At equal ion mobility for each sulfate anion, there are two nitrate ions to neutralize a cationic species, for example the copper (II) ion (Cu2+), but with a great chemical difference: nitrates are one hundred percent soluble in water; however, not all sulfates are water-soluble. This is the material difference that explains why the presence of nitrate salts in ore deposits is so beneficial. Additionally, this explains the low consumption rate of sulfuric acid required during leaching when the deposits contain nitrate salts.
In the matter of heap leaching of metal sulfides, it is highly desirable to apply the experience obtained from copper oxide (CuO) leaching to the copper sulfide (CuS) leaching, based on the following premise. Heap leaching of mixed ores and secondary copper sulfides uses water, sulfuric acid and bacteria to dissolve the useful species. Bacterial leaching is essentially based on the ability of microorganisms to oxidize the iron (II) (Fe2+) content in raffinate solutions (also known as raffinate) to iron (III) (Fe3+) as a result of metabolic processes. In bacterial leaching of copper sulfides and copper mixed ores, the electrochemical couple or redox pair Fe3+/Fe2+ provides the redox conditions in the galvanic cell which allows for dissolution of useful species via an electrochemical process as what occurs in copper oxide leaching. In this way, the microbes catalyze, through their metabolic processes, the oxidation of Fe2+to Fe3+. It is important to bear in mind that the electrochemical system Fe3+/Fe2+ is reversible and therefore, the oxide-reduction kinetics is very fast on conductive substrates. This process, which appears to be very efficient and simple, depends to a great extent on the bacteria concentration in heaps and the bacterial activity which, in turn, depends on (i) environmental factors, such as temperature, acidity, air pressure, and passivating chemicals, and (ii) the chemical composition of the relevant heap structure. Moreover, in the case where the water used is seawater, there are more passivating chemicals which would hinder the bacterial activity.
Chilean patent CL43295 to Jaime Arias (“Arias Patent”) discloses a method for extracting copper or other metals from sulfide ores, without the addition of oxygen gas (O2) to the ore and without the production of NOx gases, the method comprising the following steps:
Crushing the ore to a size smaller than 6 microns (0.006 m);
Treating such ore with concentrated sulfuric acid to agglomerate the fine particles and impregnate the ore with acid;
Stacking of treated ore in a heap of less than 12 m in height;
Irrigating the heap with dilute aqueous solution consisting of sulfuric acid and sodium nitrate (NaNO3) to obtain the leaching product;
Collecting the leaching product from the heap; and
Recovering the copper metal from that leaching product by electrolysis.
The Arias Patent, however, is restricted to sulfide leaching using sodium nitrate and not to the preparation of a reagent to be used in leaching.
The research study conducted by M. C. Fuerstenau, C. O. Nebo, B. V. Elango, and K. N. Han, entitled The Kinetics of Leaching Galena with Ferric Nitrate, studied the leaching rate of a sulfide ore, i.e. galena, adding an aqueous ferric nitrate (Fe(NO3)3) solution with a concentration of 0.25 M [mol/L]. The study does not show how to obtain the ferric nitrate.
The U.S. Pat. No. 4,649,029 to Inderjit Nidrosh discloses a method for extracting uranium (U) and radium (Ra) from sulfide ores and incorporating ferric nitrate in the leaching process, without mentioning how to obtain ferric nitrate. The process is comprised of the following steps:
Crush the ore finely to remove the sulfide therefrom;
Leach the finely ground ore with aqueous acidic ferric nitrate solution in a concentration range of 0.01 to 0.1 M for the removal of uranium and radium;
Separate uranium and radium from the resulting leaching solution; and
Recover the ferric nitrate, uranium, and radium from the leached ore.
PCT Application WO 2012/162851 A1 [PCT/CL2012/000022] to Alejandro Cartagena Fagerström and Christian Hecker Cartes discloses a method of generating ferric nitrate as a oxidizing and dissolving agent for copper species, especially in copper sulfides from the concentration of iron from a copper raffinate solution of a hydrometallurgical process but does not provide for the dosing of ferric nitrate to stages of the copper sulfide ore hydrometallurgical process in which the very ferric nitrate reagent is produced.